Scientists have long sought efficient ways to convert methane and other alkanes into higher hydrocarbons, including light olefins and gasoline-range materials. Efficient processes could create value in a number of ways, including: facilitating the utilization of remotely located stranded natural gas through its conversion into more easily transportable liquid fuels and feedstocks, and allowing the use of inexpensive feedstocks (methane and other lower alkanes) for end products often made from higher alkanes, including ethylene and propylene.
U.S. Pat. Nos. 6,486,368, 6,472,572, 6,465,699, 6,465,696, and 6,462,243 disclose processes for converting alkanes into olefins, ethers, and alcohols. Many of the disclosed processes involve halogenation of an alkane, passing the halogenated products over a metal oxide to create products and metal halide, recovering the product(s), and regenerating the metal halide with oxygen or air to yield metal oxide and halogen for recycle to the process. Not described is alkane oligomerization: substantial coupling of the starting hydrocarbon to obtain product(s) of higher carbon number.
Several investigators have examined the use of halogenation for the production of higher hydrocarbons from methane. Representative patents include U.S. Pat. No. 4,513,092 (Chu), U.S. Pat. No. 4,769,504 (Noceti and Taylor), U.S. Pat. No. 5,087,786 (Nubel), and U.S. Pat. No. 6,452,058 (Schweitzer). As described in the Taylor patent: “Aromatic-rich, gasoline boiling range hydrocarbons [are made] from the lower alkanes, particularly from methane. The process is carried out in two stages. In the first, alkane is reacted with oxygen and hydrogen chloride over an oxyhydrochlorination catalyst such as copper chloride with minor proportions of potassium chloride and rare earth chloride. This produces an intermediate gaseous mixture containing water and chlorinated alkanes. The chlorinated alkanes are contacted with a crystalline aluminosilicate catalyst in the hydrogen or metal-promoted form to produce gasoline range hydrocarbons with a high proportion of aromatics and a small percentage of light hydrocarbons (C2-C4), as well as reforming the HCl. The light hydrocarbons can be recycled for further processing over the oxyhydrochlorination catalyst.” All of these techniques for making higher alkanes from C1 feedstocks suffer from the disadvantage that the hydrocarbon stream must be separated from an aqueous hydrohalic acid stream, and the hydrohalic acid stream must be recycled.
U.S. Pat. No. 4,795,843 (Tomotsu et al.) discloses a process for oligomerizing halomethanes to products including ethyl benzene, toluene, and xylenes, using silica polymorph or silicalite catalysts. The process does not incorporate reactive neutralization of hydrogen halide, and appears to suffer from slow kinetics.
In a process for halogenating hydrocarbons, Chang and Perkins noted trace amounts of oligomerization products in the presence of zeolites in U.S. Pat. No. 4,654,449. The oligomerization products were low in quantity, and generally halogenated.
U.S. Pat. No. 4,373,109 (Olah) discloses a process for converting heterosubstituted methanes, including methyl halides, by contacting such methanes with bifunctional acid-base catalysts at elevated temperatures, between 200 and 450 C, preferably between 250 and 375 C, to produce predominantly lower olefins, preferably ethylene and propylene. The catalysts of preference are those derived from halides, oxyhalides, oxides, sulfides or oxysulfides of transition metals of Groups IV, V, VI, VIII of the Periodic Table, such as tantalum, niobium, zirconium, tungsten, titanium, and chromium, deposited on acidic oxides and sulfides such as alumina, silica, zirconia or silica-alumina. Neither the use of solid oxide-based halogen recovery nor the formation of alcohols or ethers is disclosed. A related reference is “Ylide chemistry. 1. Bifunctional acid-base-catalyzed conversion of heterosubstituted methanes into ethylene and derived hydrocarbons. The oniuin-ylide mechanism of the C1→C2 conversion” by George A. Olah et al. (J. Am. Chem. Soc. 106, 2143 (1984)).
U.S. Pat. No. 3,894,107 (Butter, et al.) discloses improvements to a process for condensing halogenated hydrocarbons using zeolite catalysts. Notably absent is any discussion of solid oxide-based hydrogen halide neutralization.
Kochi has observed reductive coupling of alkyl halides when transition metal bromides are reacted with low-molecular weight Grignard reagents in THF or diethyl ether (Bulletin of the Chemical Society of Japan v. 44 1971 pp. 3063-73). Liquid phase chemistry, however, typically suffers from such disadvantages as the requirement of solvent, corrosion, and lower rates of reaction than gas-phase chemistry. In addition, such a process consumes energy required to produce the magnesium metal needed for the energetic and reducing Grignard reagents. This is not the same type of process as the dehydrohalogenative coupling and hydrogen halide neutralization we describe herein.